Single ply roof guarantees usually last 15-20 years, but the real service life...

https://daisydustmaid.com/overlay-vs-full-replacement-for-a-single-ply-roof-when-does-overlay-make-sense/

Single ply roof guarantees usually last 15-20 years, but the real service life can stretch to 25-40 years if the roof’s looked after properly. These guarantees cover materials and workmanship but don’t always account for wear and tear or poor maintenance